Background and objectives: A major obstacle to the successful use of chemotherapy in the treatment of leukemia and other cancers is the emergence of drug resistance. One of the most studied resistance mechanisms is mediated by P-glycoprotein, which can be modulated by calcium channel blockers. Here we investigated whether the Ca(2+) channel blockers verapamil and nifedipine are toxic alone and in combination with P-glycoprotein-independent anticancer drugs against chronic lymphocytic leukemia (CLL) cells in vitro.
Design and methods: Verapamil cytotoxicity was investigated in peripheral blood samples of 35 patients with B-cell CLL and 10 healthy control subjects. Cytotoxicity was assessed in in vitro 4-day cultures using 14C-leucine incorporation as an indicator of cell viability. Interactions were tested with Ca2+ channel blockers and cyclosporine or 7 anticancer drugs: (i) chlorambucil, (ii) 2-chlorodeoxyadenosine, (iii) cisplatin, (iv) fludarabine, (v) prednisolone, (vi) adriamycin, and (vii) vincristine. The mode of cell death was assessed by annexin binding and DNA ladder formation.
Results: Verapamil induced dose- and time -dependent death of CLL cells in vitro. A statistically significant effect (p = 0.0085) was noted with as little as 4 microM verapamil. The mode of cell death was apoptotic as determined by annexin positivity and condensation of verapamil-treated cells. Verapamil effectively potentiated the toxicity of cyclosporine and all the anticancer drugs mentioned above. Furthermore, nifedipine, a more specific L-type calcium channel antagonist, significantly potentiated the effect of chlorambucil against CLL cells. Interpretation and Conclusions. Calcium channel blockers enhance the effect of P-glycoprotein-independent anticancer drugs remarkably. This indicates that the death signals initiated by calcium depletion and anticancer drugs together facilitate cell death. This novel finding opens a new avenue to modulate, by using calcium channel antagonists, the effect of traditional anticancer drugs having different mechanisms of P-glycoprotein-independent action.